I get this very question from time to time, mostly from paranoid S&W owners. Endshake in a S&W can be devastating and will worsen considerably if not promptly taken care of (gun shoots loose). This will continue to a point where the cylinder will unlock when fired. In S&W revolvers, the bearing surface that dictates endshake is the skinny little yoke tube end. With constant use and especially hot loads, this tiny surface gets pounded down thus increasing endshake.
Rugers are a similar design, just more massive where it counts. Although a few thousandths endshake would be optimum, it doesn't hurt a thing to have as much as .006". GP-100's yokes have a much larger bearing surface so shooting a lot of full power loads won't have much effect on endshake.
Actually, GP-100s function better when there are a few thousandths endshake. When they are too tight, the cylinder/yoke's ball bearings will bind making DA trigger pull harsher.
Ruger DAs are well designed for strength and longivity. S&Ws may be more "precision" but they can't stand up to the pounding that a Ruger will take. Even the large N-frame S&Ws have the skinny little yoke tube and will shoot loose.
Most shooters think the frames on a Ruger are the reason for strength. Yes, they are a bit thicker but the real strength comes from their yoke tube's surface area which is about 4 times larger than a S&W.
Bottom line: if your cylinder gap still has a couple thousandths clearance when you hold the cylinder forward, your endshake is OK.